A few years ago, Siobhan Dowd and I were both shortlisted for a book award in Germany. Shortly before the ceremony, she died. At the venue I told my minder that Siobhan's agent had come ready with a speech in case she won. "Oh, but of course she can't win," said the minder. "The prize is for living authors. Now she is dead, so she is disqualified." "Oh but . . . she's only just dead. I mean, she was definitely alive when she wrote the book." "But now she's dead. So you have an extra chance to win."

When does a writer really die? Since Dowd's death, her publisher has brought out her Carnegie-winning novel, Solace of the Open Road. And now Patrick Ness and illustrator Jim Kay have created a new book from a set of notes that she left behind. In a moving introduction, Ness says it was like being handed a baton and told to run. Well, he ran fast and he ran with grace. A Monster Calls is the story of Conor, who is repeatedly visited by a monster while his mother is dying.

The monster is a brilliant creation – part giant, part yew tree, destructive, didactic, elemental. It tells Conor three stories, which work, like New Testament parables, by wrongfooting you. The good guys turn out to be bad and the bad guys good. Elegantly, the same goes for the overarching story, in which the nightmare monster is less frightening than daylight family. The prospect of Conor's mother's death brings not only grief and the primal fear of death itself but a list of no less terrifying pragmatic anxieties: who is going to look after me? Who can I count on? Where will I live?

The book has the thrills and ambition you would expect from the author of the Chaos Walking trilogy. It's also easy to trace Dowd's influence. There's a very Irish emphasis, for instance, on the importance of making a "good death" – with your loved ones around you and the right things said. But perhaps the most impressive thing about it is that it's nothing like Ness's other books and nothing like Dowd's. Like the monster, it has a life of its own.

It's also an extraordinarily beautiful book. Kay's menacing, energetic illustrations and the way they interact with the text, together with the lavish production values, make it a joy just to hold in your hand. If I have one quibble, it is with a line in the introduction where Ness says the point of a story is to "make trouble". It seems to me he has done the opposite here. He's produced something deeply comforting and glowing with – to use a Siobhan Dowd word – solace. The point of art and love is to try to shortchange that grim tax collector, death. Ness, Dowd, Kay and Walker have rifled death's pockets and pulled out a treasure. Death, it seems, is no disqualification.